The invention is based on a motion-damping apparatus for a vehicle. An apparatus of this type has set forth in U.S. patent aqpplication Ser. No. 800,574 filed Nov. 21, 1985, now abandoned, in which the hydraulic properties of the damping fluid can be varied by applying electrical fields. An electrorheological fluid is used as the damping fluid. The damping fluid flowing back and forth between the two work chambers of the apparatus is guided past plate electrodes; this changes its hydraulic properties and in turn makes it possible to vary the damping properties of the apparatus as desired. The higher the voltage present at the electrodes, the greater the shear stress of the electrorheological fluid at the electrode surfaces becomes, and hence the greater the damping of the apparatus.
A disadvantage of the above noted apparatus is that the damping range attainable with it is quite small, and cannot properly address all the situations that arise while driving.
Moreover, if the electrodes or their electrical triggering fails, the shearing strain of the damping voltage drops to a minimal value, namely that of the normal shear stress, so that the apparatus has the least possible damping capacity. If such a defect occurs suddenly, a vehicle equipped with the above noted apparatus can be in danger, and at least will have an unfavorable chassis characteristic for emergency operation.